Johnny, I'm about to start on a Sig SkyBolt. I've spoken with several guys who have built this one, and all said a good .61 two stroke or .91 four stroke was plenty; unless the model was a lead sled. One fellow has a Saito .65 four-stroke on his; he said it was adequate, but barely. Apparently, an overly heavy or underpowered one is a real bear to fly. This kit is an older vintage and was designed around a .61 two stroke, so that's what I'm going to use, with a Pitts muffler.